Blue color converter for micro leds

ABSTRACT

A photocurable composition includes a blue photoluminescent material, one or more monomers, and a photoinitiator that initiates polymerization of the one or more monomers in response to absorption of the ultraviolet light. The blue photoluminescent material is selected to absorb ultraviolet light with a maximum wavelength in a range of about 300 nm to about 430 nm and to emit blue light. The blue photoluminescent material also has an emission peak in a range of about 420 nm to about 480 nm. The full width at half maximum of the emission peak is less than 100 nm, and the photoluminescence quantum yield is in a range of 5% to 100%.

TECHNICAL FIELD

This disclosure generally relates to fabrications methods for blue colorconverters installed by self-aligned in-situ curing for micro-LEDs andsystems and devices including the blue color converters.

BACKGROUND

A light emitting diode (LED) panel uses an array of LEDs, withindividual LEDs providing the individually controllable pixel elements.Such an LED panel can be used for a computer, touch panel device,personal digital assistant (PDA), cell phone, television monitor, andthe like.

An LED panel that uses micron-scale LEDs based on III-V semiconductortechnology (also called micro-LEDs) would have a variety of advantagesas compared to OLEDs, e.g., higher energy efficiency, brightness, andlifetime, as well as fewer material layers in the display stack whichcan simplify manufacturing. However, there are challenges to fabricationof micro-LED panels. Micro-LEDs having different color emission (e.g.,red, green and blue pixels) need to be fabricated on differentsubstrates through separate processes. Integration of the multiplecolors of micro-LED devices onto a single panel typically requires apick-and-place step to transfer the micro-LED devices from theiroriginal donor substrates to a destination substrate. This ofteninvolves modification of the LED structure or fabrication process, suchas introducing sacrificial layers to ease die release. In addition,stringent requirements on placement accuracy can limit the throughput,the final yield, or both.

An alternative approach to bypass the pick-and-place step is toselectively deposit color conversion agents (e.g., quantum dots,nanostructures, photoluminescent materials, or organic substances) atspecific pixel locations on a substrate fabricated with monochrome LEDs.The monochrome LEDs can generate relatively short wavelength light,e.g., purple or blue light, and the color conversion agents can convertthis short wavelength light into longer wavelength light, e.g., red orgreen light for red or green pixels. The selective deposition of thecolor conversion agents can be performed using high-resolution shadowmasks or controllable inkjet or aerosol jet printing.

However, shadow masks are prone to problems with alignment accuracy andscalability, whereas inkjet and aerosol jet techniques suffer fromresolution (inkjet), accuracy (inkjet) and throughput (aerosol jet)problems. In order to manufacture micro-LED displays, new techniques areneeded to precisely and cost-effectively provide color conversion agentsfor different colors onto different pixels on a substrate, such as alarge area substrate or flexible substrate.

SUMMARY

This disclosure generally relates to fabrication methods for blue colorconverters installed by self-aligned in-situ curing for micro-LEDs andsystems and devices including the blue color converters

In a general aspect, a photocurable composition includes a bluephotoluminescent material, one or more monomers, and a photoinitiatorthat initiates polymerization of the one or more monomers in response toabsorption of the ultraviolet light. The blue photoluminescent materialis selected to absorb ultraviolet light with a maximum wavelength in arange of about 300 nm to about 430 nm and to emit blue light. The bluephotoluminescent material also has an emission peak in a range of about420 nm to about 480 nm. The full width at half maximum of the emissionpeak is less than 100 nm, and the photoluminescence quantum yield is ina range of 5% to 100%.

In another aspect, a photocured composition includes a polymer matrix, ablue photoluminescent material mixed in the polymer matrix, andcomponents of a photoinitiator that initiated polymerization to form thepolymer matrix. The blue photoluminescent material selected to absorbultraviolet light with a maximum wavelength in a range of about 300 nmto about 430 nm and to emit blue light with an emission peak in a rangeof about 420 nm to about 480 nm, wherein the full width at half maximumof the emission peak is less than 100 nm, and the photoluminescencequantum yield is in a range of 5% to 100%.

In another aspect, a method of fabricating a light emitting deviceincludes dispensing a first photo-curable fluid over a display having abackplane and an array of ultraviolet light emitting diodes electricallyintegrated with backplane circuitry of the backplane, activating a firstplurality of light emitting diodes in the array of light emitting diodesto illuminate and polymerize the one or more monomers to form a firstcolor conversion layer over each of the first plurality of lightemitting diodes to convert light from the first plurality of lightemitting diodes to blue light, each color conversion layer includingblue photoluminescent material mixed in a polymer matrix, and removingan uncured remainder of the first photo-curable fluid. The firstphoto-curable fluid includes a blue photoluminescent material selectedto absorb ultraviolet light with a maximum wavelength in a range ofabout 300 nm to about 430 nm and to emit blue light with an emissionpeak in a range of about 420 nm to about 480 nm, wherein the full widthat half maximum of the emission peak is less than 100 nm, and thephotoluminescence quantum yield is in a range of 5% to 100%, one or moremonomers, and a photoinitiator that initiates polymerization of the oneor more monomers in response to absorption of the ultraviolet light.

Implementations of these aspects may include one or more of thefollowing features.

The composition typically includes about 0.1 wt % to about 10 wt % ofthe blue photoluminescent material, about 0.5 wt % to about 5 wt % ofthe photoinitiator, and about 1 wt % to about 90 wt % of the one or moremonomers. In some cases, the composition includes a solvent. Acomposition with a solvent typically includes about 0.1 wt % to about 10wt % of the blue photoluminescent material, about 0.5 wt % to about 5 wt% of the photoinitiator, about 1 wt % to about 90 wt % of the one ormore monomers, and about 10 wt % to about 90 wt % of the solvent.

The blue photoluminescent material can be an organic material, anorganometallic material, or a polymeric material. In some cases, theblue photoluminescent material is an organic material, and the organicmaterial is a free radical. The blue photoluminescent material can befluorescent or phosphorescent.

Advantages of the blue color converters and systems and devicesincluding the blue color converters include high photoluminescencequantum yield, long lifetime, and long shelf lifetime.

The details of one or more embodiments of the subject matter of thisdisclosure are set forth in the accompanying drawings and thedescription. Other features, aspects, and advantages of the subjectmatter will become apparent from the description, the drawings, and theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows structural formulas of blue fluorescent molecules. FIG. 1Bshows structural formulas of blue thermally activated delayedfluorescent molecules. FIG. 1C shows structural formulas of bluephosphorescent organic and organometallic complexes.

FIG. 2 is a schematic top view of a micro-LED array that has alreadybeen integrated with a backplane.

FIG. 3A is a schematic top view of a portion of a micro-LED array. FIG.3B is a schematic cross-sectional view of the portion of the micro-LEDarray from FIG. 3A.

FIGS. 4A-4H illustrate a method of selectively forming color conversionlayers over a micro-LED array.

FIGS. 5A-5C illustrate formulations of photocurable composition.

FIGS. 6A-6E illustrate a method of fabricating a micro-LED array andisolation walls on a backplane.

FIGS. 7A-7D illustrate another method of fabricating a micro-LED arrayand isolation walls on a backplane.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Quantum dots can be dispersed in acrylate formulations for inkjetprinting. Followed by UV curing, the quantum dots locked in apolyacrylate matrix can be used as a color conversion layer for advanceddisplays. However, while red and green light can be achieved by quantumdots, quantum dots that convert UV light to blue light (e.g., ZeS/Se/Tequantum dots) typically suffer from low photoluminescence quantum yield(PLQY), short working lifetime, and short shelf lifetime.

A technique that may address problems associated with the lack of bluecolor conversion agents for micro-LEDs with UV backlight includes usingmaterials other than quantum dots for blue color conversion agents.These blue color conversion agents can be included in a formulation fora micro-LED blue color conversion layer formed by self-aligned curing asdescribed in this disclosure.

A formulation for a micro-LED blue color conversion layer typicallyincludes a blue color conversion agent (e.g., free of quantum dots), areactive component, and a photoinitiator. The formulation may optionallyinclude one or more of a solvent, a functional component (e.g., highrefraction index additive, a surfactant, a stray light absorber or UVblocker).

Suitable blue color conversion agents include fluorescent andphosphorescent organic molecules, organic radicals, organometalliccomplexes, and polymers including one or more of these color conversionagents. Blue color conversion agents are selected to have strongabsorption of UV light with a maximum wavelength (λ_(max)) in a range ofabout 300 nm to about 430 nm and to emit blue light with an emissionpeak in a range of about 420 nm to about 480 nm. The full width at halfmaximum (FWHM) the emission peak is typically less than 100 nm, and thephotoluminescence quantum yield (PLQY) is typically in a range of 5% to100%.

Examples of suitable blue color conversion agents include LUMILUX BlueCD 310, LUMILUX Blue CD 710, and LUMILUX Dispersion Blue CD 910(available from Honeywell International Inc.). FIG. 1A shows structuralformulas of the blue fluorescent molecules 4P-NPD, Bepp₂, TPA-SBFF,DPAFVF, Ban-(3,5)-CF3, TBPe, DBzA, BITPI, BiPI-1, 4PF, TPI-Py, andPhImA. FIG. 1B shows structural formulas of blue thermally activateddelayed fluorescent (TADF) molecules v-DABNA, DMAC-DPS, CZ-PS, DMTDAc,DMAC-TRZ, Cab-Ph-TRZ, Ca-TRZ2, Cz-TRZ3, Cz-TRZ4, BCC-TPTA, DDCzTrz,DPCC-TPTA, DCzTrz, BDPCC-TPTA, Phen-TRZ, TCzTrz, Cz-VPN, CPC, 2PXZ-TAZ,and CC2BP. FIG. 1C shows structural formulas of blue phosphorescentorganic and organometallic complexes including a metalloid (boron) andmetals (beryllium and iridium).

Formulations for micro-LED red and green color conversion layerstypically include a red or green color conversion agent, respectively, areactive component, and a photoinitiator. The formulation may optionallyinclude one or more of a solvent, a functional component (e.g., highrefraction index additive, a surfactant, a stray light absorber or UVblocker).

The red and green color conversion agents are materials that emitvisible radiation in a first visible wavelength band in response toabsorption of UV radiation or visible radiation in a second visiblewavelength band. The UV radiation typically has a wavelength in a rangeof 200 nm to 400 nm. The visible radiation typically has a wavelength orwavelength band in a range of 500 nm to 800 nm. The first visiblewavelength band is different (e.g., more energetic) than the secondvisible wavelength band. That is, the color conversion agents arematerials that can convert the shorter wavelength light from a micro-LEDinto longer wavelength light.

The red and green color conversion agents can include photoluminescentmaterials, such as organic or inorganic molecules, nanomaterials (e.g.,nanoparticles, nanostructures, quantum dots), or other appropriatematerials. Suitable nanomaterials typically include one or more III-Vcompounds. Examples of suitable III-V compounds include CdSe, CdS, InP,PbS, CuInP, ZnSeS, and GaAs. In some cases, the nanomaterials includeone or more elements selected from the group consisting of cadmium,indium, copper, silver, gallium, germanium, arsenide, aluminum, boron,iodide, bromide, chloride, selenium, tellurium, and phosphorus. Incertain cases, the nanomaterials include one or more perovskites.

The quantum dots can be homogeneous or can have a core-shell structure.The quantum dots can have an average diameter in a range of about 1 nmto about 10 nm. One or more organic ligands are typically coupled to anexterior surface of the quantum dots. The organic ligands promotedispersion of the quantum dots in solvents. Suitable organic ligandsinclude aliphatic amine, thiol or acid compounds, in which the aliphaticpart typically has 6 to 30 carbon atoms. Examples of suitablenanostructures include nanoplatelets, nanocrystals, nanorods, nanotubes,and nanowires.

The reactive components include monomers, such as (meth)acrylatemonomers, and can include one or more mono(meth)acrylates,di(meth)acrylates, tri(meth)acrylates, tetra(meth)acrylates, or acombination thereof. The reactive components can be provided by anegative photoresist, e.g., SU-8 photoresist. Examples of suitablemono(meth)acrylates include isobornyl (meth)acrylates, cyclohexyl(meth)acrylates, trimethylcyclohexyl (meth)acrylates, diethyl(meth)acrylamides, dimethyl (meth)acrylamides, and tetrahydrofurfuryl(meth)acrylates. The reactive component may include cross-linkers orother reactive compounds. Examples of suitable cross-linkers includepolyethylene glycol di(meth)acrylates (e.g., diethylene glycoldi(meth)acrylate or tripropylene glycol di(meth)acrylates),N,N′-methylenebis-(meth)acrylamides, pentaerythritol tri(meth)acrylates,and pentaerythritol tetra(meth)acrylates. Examples of suitable reactivecompounds include polyethylene glycol (meth)acrylates, vinylpyrrolidone,vinylimidazole, styrenesulfonate, (meth)acrylamides,alkyl(meth)acrylamides, dialkyl(meth)acrylamides),hydroxyethyl(meth)acrylates, morpholinoethyl acrylates, andvinylformamides.

The photoinitiator initiates polymerization in response to radiationsuch as UV radiation, UV-LED radiation, visible radiation, and electronbeam radiation. In some cases, the photoinitiator is responsive to UV orvisible radiation. Suitable photoinitiators include free radicalphotoinitiators, such as bulk cure photoinitiators and surface curephotoinitators.

Bulk cure photoinitiators cleave upon exposure to UV radiation, yieldinga free radical, which may initiate polymerization. Bulk curephotoinitiators can be useful for both surface and through or bulk cureof the dispensed droplets. Bulk cure photoinitiators include benzoinethers, benzyl ketals, acetyl phenones, alkyl phenones, phosphineoxides, benzophenone compounds, and thioxane compounds.

Surface cure photoinitiators are activated by UV radiation and form freeradicals by hydrogen abstraction from a second compound, which becomesthe actual initiating free radical. This second compound is often calleda co-initiator or polymerization synergist, and may be an aminesynergist. Amine synergists are used to diminish oxygen inhibition, andtherefore, surface cure photoinitiators can be useful for fast surfacecures. Surface cure photoinitiators include benzophenone compounds andthioxanthone compounds. An amine synergist is an amine with an activehydrogen. Amine synergists, such as amine-containing acrylates, may becombined with a benzophenone photoinitiator in a resin precursorcomposition formulation to: a) limit oxygen inhibition, b) fast cure adroplet or layer surface so as to fix the dimensions of the droplet orlayer surface, and c), increase layer stability through the curingprocess.

Examples of suitable photoinitiators include 1-hydroxycyclohexylphenylketone, 4-isopropylphenyl-2-hydroxy-2-methyl propan-1-one,1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propan-1-one,2,2-dimethyl-2-hydroxy-acetophenone, 2,2-dimethoxy-2-phenylacetophenone,2-hydroxy-2-methylpropionphenone, diphenyl (2,4,6-trimethylbenzoyl)phosphine oxide, bis(2,6-dimethoxy-benzoyl)-2,4,6 trimethyl phenylphosphine oxide,2-methyl-1-1[4-(methylthio)phenyl]-2-morpholino-propan-1-one,3,6-bis(2-methyl-2-morpholino-propionyl)-9-n-octylcarbazole,2-benzyl-2-(dimethylamino)-1-(4-morpholinyl)phenyl)-1-butanone,benzophenone, 2,4,6-trimethylbenzophenone, isopropyl thioxanthone,phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide,2-hydroxy-2-methyl-1phenyl-1-propanone. Suitable blends ofphotoinitiators commercially available include Darocur 4265, Irgacure184, Irgacure 250, Irgacure 270, Irgacure 295, Irgacure 369, Irgacure379, Irgacure 500, Irgacure 651, Irgacure 754, Irgacure 784, Irgacure819, Irgacure 907, Irgacure 1173, Irgacure 2100, Irgacure 2022, Irgacure4265, Irgacure TPO, Irgacure TPO-L, Esacure KT37, Esacure KT55, EsacureKT0046, Omnicat 250, and Omnicat 550. Suitable amine synergists includesecondary and tertiary amine compounds with or without acrylic groups,such as diethanolamine, triethanolamine, and Genomer 5142.

Optionally, the photocurable composition can include a solvent. Thesolvent can be organic or inorganic. Examples of suitable solventsinclude water, ethanol, toluene, dimethylformamide, methylethylketone,or a combination thereof. The solvent can be selected to provide adesired surface tension or viscosity for the photocurable composition.The solvent can also improve chemical stability of the other components.

Optionally, the photocurable composition can include a straylightabsorber or a UV blocker. Examples of suitable straylight absorbersinclude Disperse Yellow 3, Disperse Yellow 7, Disperse Orange 13,Disperse Orange 3, Disperse Orange 25, Disperse Black 9, Disperse Red 1acrylate, Disperse Red 1 methacrylate, Disperse Red 19, Disperse Red 1,Disperse Red 13, and Disperse Blue 1. Examples of suitable UV blockersinclude benzotriazolyl hydroxyphenyl compounds.

Optionally, the first photocurable composition can include one or moreother functional ingredients. As one example, the functional ingredientscan affect the optical properties of the color conversion layer. Forexample, the functional ingredients can include nanoparticles with asufficiently high index of refraction (e.g., at least about 1.7) thatthe color conversion layer functions as an optical layer that adjuststhe optical path of the output light, e.g., provides a microlens.Examples of suitable nanoparticles include TiO₂, ZnO₂, ZrO₂, CeO₂, or amixture of two or more of these oxides. Alternatively or in addition,the nanoparticles can have an index of refraction selected such that thecolor conversion layer functions as an optical layer that reduces totalreflection loss, thereby improving light extraction. As another example,the functional ingredients can include a dispersant or surfactant toadjust the surface tension of the photocurable composition. Examples ofsuitable dispersants or surfactants include siloxane and polyethyleneglycol. As yet another example, the functional ingredients can include aphotoluminescent pigment that emits visible radiation. Examples ofsuitable photoluminescent pigments include zinc sulfide and strontiumaluminate.

In some cases, the photocurable composition includes about up to about90 wt % of the reactive component (e.g., about 10 wt % to about 90 wt%), about 0.5 wt % to about 5 wt % of a photoinitiator, and about 0.1 wt% to about 10 wt % (e.g., about 1 wt % to about 2 wt %) of a colorconversion agent. The photocurable composition may also include asolvent (e.g., up to about 10 wt % of a solvent).

The photocurable composition can optionally include up to about 5 wt %of a surfactant or dispersant, about 0.01 wt % to about 5 wt % (e.g.,about 0.1 wt % to about 1 wt %) of a straylight absorber, or anycombination thereof.

A viscosity of the photocurable composition is typically in a range ofabout 10 cP (centiPoise) to about 2000 cP at room temperature (e.g.,about 10 cP to about 150 cP). A surface tension of the photocurablecomposition is typically in a range of about 20 milliNewtons per meter(mN/m) to about 60 mN/m (e.g., about 40 mN/m to about 60 mN/m). Aftercuring, an elongation at break of the cured photocurable composition istypically in a range of about 1% to about 200%. A tensile strength ofthe cured photocurable composition is typically in a range of about 1megaPascal (MPa) to about 1 gigaPascal (GPa). The photocurablecomposition can be applied in one or more layers, and a thickness of thecured photocurable composition is typically in a range of about 10 nm toabout 100 microns (e.g., about 10 nm to about 20 microns, about 10 nm toabout 1000 nm, or about 10 nm to about 100 nm).

The photocurable compositions described in this disclosure areimplemented as color conversion layers in displays, such as micro-LEDdisplays described with respect to FIGS. 2-7.

FIG. 2 illustrates a micro-LED display 10 that includes an array 12 ofindividual micro-LEDs 14 (see FIGS. 3A and 3B) disposed on a backplane16. The micro-LEDs 14 are already integrated with backplane circuitry 18so that each micro-LED 14 can be individually addressed. For example,the backplane circuitry 18 can include a TFT active matrix array with athin-film transistor and a storage capacitor (not illustrated) for eachmicro-LED, column address and row address lines 18 a, column and rowdrivers 18 b, etc., to drive the micro-LEDs 14. Alternatively, themicro-LEDs 14 can be driven by a passive matrix in the backplanecircuitry 18. The backplane 16 can be fabricated using conventional CMOSprocesses.

FIGS. 3A and 3B illustrate a portion 12 a of the micro-LED array 12 withthe individual micro-LEDs 14. All of the micro-LEDs 14 are fabricatedwith the same structure so as to generate the same wavelength range(this can be termed “monochrome” micro-LEDs). For example, themicro-LEDs 14 can generate light in the ultraviolet (UV), e.g., the nearultraviolet, range. For example, the micro-LEDs 14 can generate light ina range of 365 to 405 nm. As another example, the micro-LEDs 14 cangenerate light in the violet or blue range. The micro-LEDs can generatelight having a spectral bandwidth of 20 to 60 nm.

FIG. 3B illustrates a portion of the micro-LED array that can provide asingle pixel. Assuming the micro-LED display is a three-color display,each pixel includes three sub-pixels, one for each color, e.g., one eachfor the blue, green and red color channels. As such, the pixel caninclude three micro-LEDs 14 a, 14 b, 14 c. For example, the firstmicro-LED 14 a can correspond to a blue subpixel, the second micro-LED14 b can correspond to a green subpixel, and the third micro-LED 14 ccan correspond to a red subpixel. However, the techniques discussedbelow are applicable to micro-LED displays that use a larger number ofcolors, e.g., four or more colors. In this case, each pixel can includefour or more micro-LEDs, with each micro-LED corresponding to arespective color. In addition, the techniques discussed below areapplicable to micro-LED displays that use just two colors.

In general, the monochrome micro-LEDs 14 can generate light in awavelength range having a peak with a wavelength no greater than thewavelength of the highest-frequency color intended for the display,e.g., purple or blue light. The color conversion agents can convert thisshort wavelength light into longer wavelength light, e.g., red or greenlight for red or green subpixels. If the micro-LEDs generate UV light,then color conversion agents can be used to convert the UV light intoblue light for the blue subpixels.

Vertical isolation walls 20 are formed between neighboring micro-LEDs.The isolation walls provide for optical isolation to help localizepolymerization and reduce optical crosstalk during the in-situpolymerization discussed below. The isolation walls 20 can be aphotoresist or metal, and can be deposited by conventional lithographyprocesses. As shown in FIG. 3A, the walls 20 can form a rectangulararray, with each micro-LED 14 in an individual recess 22 defined by thewalls 20. Other array geometries, e.g., hexagonal or offset rectangulararrays, are also possible. Possible processes for back-plane integrationand isolation wall formation are discussed in more detail below.

The walls can have a height H of about 3 to 20 μm. The walls can have awidth W of about 2 to 10 μm. The height H can be greater than the widthW, e.g., the walls can have an aspect ratio of 1.5:1 to 5:1. The heightH of the wall is sufficient to block light from one micro-LED fromreaching an adjacent micro-LED.

FIGS. 4A-4H illustrate a method of selectively forming color conversionlayers over a micro-LED array. Initially, as shown in FIG. 4A, a firstphotocurable composition 30 a is deposited over the array of micro-LEDs14 that are already integrated with the backplane circuitry. The firstphotocurable composition 30 a can have a depth D greater than a height Hof the isolation walls 20.

Referring to FIGS. 5A-5C, a photocurable composition (e.g., firstphotocurable composition 30 a, second photocurable composition 30 b,third photocurable composition 30 c, etc.) includes a polymerizablecomponents 32, a photoinitiator 34 to trigger polymerization underillumination of a wavelength corresponding to the emission of themicro-LEDs 14, and color conversion agents 36 a. The polymerizablecomponent 32 includes a reactive component and an anti-oxygen inhibitionadditive as described herein.

After curing of the photocurable composition, components of thephotoinitiator 34 may be present in the cured photocurable composition(the photopolymer), where the components are fragments of thephotoinitiator formed during breaking of bonds in the photoinitiator inthe photo-initiation process.

Returning to FIG. 4A, the first photocurable composition 30 a can bedeposited on the display over the micro-LED array by a spin-on, dipping,spray-on, or inkjet process. An inkjet process can be more efficient inconsumption of the first photocurable composition 30 a.

Next, as shown in FIG. 4B, the circuitry of the backplane 16 is used toselectively activate a first plurality of micro-LEDs 14 a. This firstplurality of micro-LEDs 14 a correspond to the sub-pixels of a firstcolor. In particular, the first plurality of micro-LEDs 14 a correspondto the sub-pixels for the color of light to be generated by the colorconversion agents in the photocurable composition 30 a. For example,assuming the color conversion agents in the fluid 30 a will convertlight from the micro-LED 14 into blue light, then only those micro-LEDs14 a that correspond to blue sub-pixels are turned on. Because themicro-LED array is already integrated with the backplane circuitry 18,power can be supplied to the micro-LED display 10 and control signalscan be applied by a microprocessor to selectively turn on the micro-LEDs14 a.

Referring to FIGS. 4B and 4C, activation of the first plurality ofmicro-LEDs 14 a generates illumination A (see FIG. 4B) which causesin-situ curing of the first photocurable composition 30 a to form afirst solidified color conversion layer 40 a (see FIG. 4C) over eachactivated micro-LED 14 a. In short, the fluid 30 a is cured to formcolor conversion layers 40 a, but only on the selected micro-LEDs 14 a.For example, a color conversion layer 40 a for converting to blue lightcan be formed on each micro-LED 14 a.

In some implementations, the illumination from the selected micro-LEDs14 a does not reach the other micro-LEDs 14 b, 14 c. In thiscircumstance, the isolation walls 20 may not be necessary. However, ifthe spacing between the micro-LEDs 14 is sufficiently small, isolationwalls 20 can affirmatively block illumination A from the selectedmicro-LED 14 a from reaching the area over the other micro-LEDs thatwould be within the penetration depth of the illumination from thoseother micro-LEDs. Isolation walls 20 can also be included, e.g., simplyas insurance against illumination reaching the area over the othermicro-LEDs.

The driving current and drive time for the first plurality of micro-LEDs14 a can be selected for appropriate photon dosage for the photocurablecomposition 30 a. The power per subpixel for curing the fluid 30 a isnot necessarily the same as the power per subpixel in a display mode ofthe micro-LED display 10. For example, the power per subpixel for thecuring mode can be higher than the power per subpixel for the displaymode.

Referring to FIG. 4D, when curing is complete and the first solidifiedcolor conversion layer 40 a is formed, the residual uncured firstphotocurable composition is removed from the display 10. This leaves theother micro-LEDs 14 b, 14 c, exposed for the next deposition steps. Insome implementations, the uncured first photocurable composition 30 a issimply rinsed from the display with a solvent, e.g., water, ethanol,toluene, dimethylformamide, or methylethylketone, or a combinationthereof. If the photocurable composition 30 a includes a negativephotoresist, then the rinsing fluid can include a photoresist developerfor the photoresist.

Referring to FIGS. 4E and 5B, the treatment described above with respectto FIGS. 4A-4D is repeated, but with a second photocurable composition30 b and activation of a second plurality of micro-LEDs 14 b. Afterrinsing, a second color conversion layer 40 b is formed over each of thesecond plurality of micro-LEDs 14 b.

The second photocurable composition 30 b is similar to the firstphotocurable composition 30 a, but includes color conversion agents 36 bto convert the shorter wavelength light from the micro-LEDs 14 intolonger wavelength light of a different second color. The second colorcan be, for example, green.

The second plurality of micro-LEDs 14 b correspond to the sub-pixels ofa second color. In particular, the second plurality of micro-LEDs 14 bcorrespond to the sub-pixels for the color of light to be generated bythe color conversion agents in the second photocurable composition 30 b.For example, assuming the color conversion agents in the fluid 30 a willconvert light from the micro-LED 14 into green light, then only thosemicro-LEDs 14 b that correspond to green sub-pixels are turned on.

Referring to FIGS. 4F and 5C, optionally the treatment described abovewith respect to FIGS. 4A-4D is repeated yet again, but with a thirdphotocurable composition 30 c and activation of a third plurality ofmicro-LEDs 14 c. After rinsing, a third color conversion layer 40 c isformed over each of the third plurality of micro-LEDs 14 c.

The third photocurable composition 30 c is similar to the firstphotocurable composition 30 a, but includes color conversion agents 36 cto convert the shorter wavelength light from the micro-LEDs 14 intolonger wavelength light of a different third color. The third color canbe, for example, red.

The third plurality of micro-LEDs 14 c correspond to the sub-pixels of athird color. In particular, the third plurality of micro-LEDs 14 ccorrespond to the sub-pixels for the color of light to be generated bythe color conversion agents in the third photocurable composition 30 c.For example, assuming the color conversion agents in the fluid 30 c willconvert light from the micro-LED 14 into red light, then only thosemicro-LEDs 14 c that correspond to red sub-pixels are turned on.

In this specific example illustrated in FIGS. 4A-4F, color conversionlayers 40 a, 40 b, 40 c are deposited for each color sub-pixel. This isneeded, e.g., when the micro-LEDs generate ultraviolet light.

However, the micro-LEDs 14 could generate blue light instead of UVlight. In this case, the coating of the display 10 by a photocurablecomposition containing blue color conversion agents can be skipped, andthe process can be performed using the photocurable compositions for thegreen and red subpixels. One plurality of micro-LEDs is left without acolor conversion layer, e.g., as shown in FIG. 4E. The process shown byFIG. 4F is not performed. For example, the first photocurablecomposition 30 a could include green CCAs and the first plurality 14 aof micro-LEDs could correspond to the green subpixels, and the secondphotocurable composition 30 b could include red CCAs and the secondplurality 14 b of micro-LEDs could correspond to the red subpixels.

Assuming that the fluids 30 a, 30 b, 30 c included a solvent, somesolvent may be trapped in the color conversion layers 40 a, 40 b, 40 c.Referring to FIG. 4G, this solvent can be evaporated, e.g., by exposingthe micro-LED array to heat, such as by IR lamps. Evaporation of thesolvent from the color conversion layers 40 a, 40 b, 40 c can result inshrinking of the layers so that the final layers are thinner.

Removal of the solvent and shrinking of the color conversion layers 40a, 40 b, 40 c can increase concentration of color conversion agents,e.g., quantum dots, thus providing higher color conversion efficiency.On the other hand, including a solvent permits more flexibility in thechemical formulation of the other components of the photocurablecompositions, e.g., in the color conversion agents or cross-linkablecomponents.

Optionally, as shown in FIG. 4H, a UV blocking layer 50 can be depositedon top of all of the micro-LEDs 14. The UV blocking layer 50 can blockUV light that is not absorbed by the color conversion layers 40. The UVblocking layer 50 can be a Bragg reflector, or can simply be a materialthat is selectively absorptive to UV light (e.g., a benzotriazolylhydroxyphenyl compound). A Bragg reflector can reflect UV light backtoward the micro-LEDs 14, thus increasing energy efficiency. Otherlayers, such as straylight absorbing layers, photoluminescent layers,and high refractive index layers include materials may also beoptionally deposited on micro-LEDs 14.

Thus, as described herein, a photocurable composition includes colorconversion agents selected to emit radiation in a first wavelength bandin the visible light range in response to absorption of radiation in asecond wavelength band in the UV or visible light range, a reactivecomponent (e.g., one or more monomers), and a photoinitiator thatinitiates polymerization of the active component in response toabsorption of radiation in the second wavelength band. The secondwavelength band is different than the first wavelength band.

In some implementations, a light-emitting device includes a plurality oflight-emitting diodes, and a cured composition in contact with a surfacethrough which radiation in a first wavelength band in the UV or visiblelight range is emitted from each of the light-emitting diodes. The curedcomposition includes a nanomaterial selected to emit radiation in asecond wavelength band in the visible light range in response toabsorption of radiation in the first wavelength band from each of thelight-emitting diodes, a photopolymer, and components (e.g., fragments)of a photoinitiator that initiates polymerization of the photopolymer inresponse to absorption of radiation in the first wavelength band. Thesecond wavelength band is different than the first wavelength band.

In certain implementations, a light-emitting device includes anadditional plurality of light-emitting diodes and an additional curedcomposition in contact with a surface through which radiation in thefirst wavelength band is emitted from each of the additionallight-emitting diodes. The additional cured composition includes anadditional CCA selected to emit radiation in a third wavelength band inthe visible light range in response to absorption of radiation in thefirst wavelength band from each of the light-emitting diodes, anadditional photopolymer, and components of an additional photoinitiatorthat initiates polymerization of the photopolymer in response toabsorption of radiation in the first wavelength band. The thirdwavelength band can be different than the second wavelength band.

FIGS. 6A-6E illustrate a method of fabricating a micro-LED array andisolation walls on a backplane. Referring to FIG. 6A, the process startswith the wafer 100 that will provide the micro-LED array. The wafer 100includes a substrate 102, e.g., a silicon or a sapphire wafer, on whichare disposed a first semiconductor layer 104 having a first doping, anactive layer 106, and a second semiconductor layer 108 having a secondopposite doping. For example, the first semiconductor layer 104 can bean n-doped gallium nitride (n-GaN) layer, the active layer 106 can be amultiple quantum well (MQW) layer 106, and the second semiconductorlayer 107 can be a p-doped gallium nitride (p-GaN) layer 108.

Referring to FIG. 6B, the wafer 100 is etched to divide the layers 104,106, 108 into individual micro-LEDs 14, including the first, second, andthird plurality of micro-LEDs 14 a, 14 b, 14 c that correspond to thefirst, second, and third colors. In addition, conductive contacts 110can be deposited. For example, a p-contact 110 a and an n-contact 110 bcan be deposited onto the n-GaN layer 104 and p-GaN layer 108,respectively.

Similarly, the backplane 16 is fabricated to include the circuitry 18,as well as electrical contacts 120. The electrical contacts 120 caninclude first contacts 120 a, e.g., drive contacts, and second contacts120 b, e.g., ground contacts.

Referring to FIG. 6C, the micro-LED wafer 100 is aligned and placed incontact with the backplane 16. For example, the first contacts 110 a cancontact the first contacts 120 a, and the second contacts 110 b cancontact the second contacts 120 b. The micro-LED wafer 100 could belowered into contact with the backplane, or vice-versa.

Next, referring to FIG. 6D, the substrate 102 is removed. For example, asilicon substrate can be removed by polishing away the substrate 102,e.g., by chemical mechanical polishing. As another example, a sapphiresubstrate can be removed by a laser liftoff process.

Finally, referring to FIG. 6E, the isolation walls 20 are formed on thebackplane 16 (to which the micro-LEDs 14 are already attached). Theisolation walls can be formed by a conventional process such asdeposition of photoresist, patterning of the photoresist byphotolithography, and development to remove the portions of thephotoresist corresponding to the recesses 22. The resulting structurecan then be used as the display 10 for the processed described for FIGS.4A-H4.

FIGS. 7A-7D illustrate another method of fabricating a micro-LED arrayand isolation walls on a backplane. This process can be similar to theprocess discussed above for FIGS. 6A-6E, except as noted below.

Referring to FIG. 7A, the process starts similarly to the processdescribed above, with the wafer 100 that will provide the micro-LEDarray and the backplane 16.

Referring to FIG. 7B, the isolation walls 20 are formed on the backplane16 (to which the micro-LEDs 14 are not yet attached).

In addition, the wafer 100 is etched to divide the layers 104, 106, 108into individual micro-LEDs 14, including the first, second and thirdplurality of micro-LEDs 14 a, 14 b, 14 c. However, the recesses 130formed by this etching process are sufficiently deep to accommodate theisolation walls 20. For example, the etching can continue so that therecesses 130 extend into the substrate 102.

Next, as shown in FIG. 7C, the micro-LED wafer 100 is aligned and placedin contact with the backplane 16 (or vice-versa). The isolation walls 20fit into the recesses 130. In addition, the contacts 110 of themicro-LEDs are electrically connected to the contacts 120 of thebackplane 16.

Finally, referring to FIG. 7D, the substrate 102 is removed. This leavesthe micro-LEDs 14 and isolation walls 20 on the backplane 16. Theresulting structure can then be used as the display 10 for the processeddescribed for FIGS. 4A-4H.

Terms of positioning, such as vertical and lateral, have been used.However, it should be understood that such terms refer to relativepositioning, not absolute positioning with respect to gravity. Forexample, laterally is a direction parallel to a substrate surface,whereas vertically is a direction normal to the substrate surface.

It will be appreciated to those skilled in the art that the precedingexamples are exemplary and not limiting. For example:

Although the above description focuses on micro-LEDs, the techniques canbe applied to other displays with other types of light emitting diodes,particularly displays with other micro-scale light emitting diodes,e.g., LEDs less than about 10 microns across.

Although the above description assumes that the order in which the colorconversion layers are formed is blue, then green, then red, other ordersare possible, e.g., blue, then red, then green. In addition, othercolors are possible, e.g., orange and yellow.

It will be understood that various modifications may be made withoutdeparting from the spirit and scope of the present disclosure.

What is claimed is:
 1. A photocurable composition comprising: a bluephotoluminescent material selected to absorb ultraviolet light with amaximum wavelength in a range of about 300 nm to about 430 nm and toemit blue light with an emission peak in a range of about 420 nm toabout 480 nm, wherein the full width at half maximum of the emissionpeak is less than 100 nm, and the photoluminescence quantum yield is ina range of 5% to 100%; one or more monomers; and a photoinitiator thatinitiates polymerization of the one or more monomers in response toabsorption of the ultraviolet light.
 2. The composition of claim 1,wherein the composition comprises: about 0.1 wt % to about 10 wt % ofthe blue photoluminescent material; about 0.5 wt % to about 5 wt % ofthe photoinitiator; and about 1 wt % to about 90 wt % of the one or moremonomers.
 3. The composition of claim 1, wherein the composition furthercomprises a solvent.
 4. The composition of claim 3, wherein thecomposition comprises: about 0.1 wt % to about 10 wt % of the bluephotoluminescent material; about 0.5 wt % to about 5 wt % of thephotoinitiator; about 1 wt % to about 90 wt % of the one or moremonomers; and about 10 wt % to about 90 wt % of the solvent.
 5. Thecomposition of claim 1, wherein the blue photoluminescent material is anorganic material, an organometallic material, or a polymeric material.6. The composition of claim 5, wherein the blue photoluminescentmaterial is an organic material, and the organic material is a freeradical.
 7. The composition of claim 5, wherein the bluephotoluminescent material is fluorescent.
 8. The composition of claim 7,wherein the blue photoluminescent material comprises blue thermallyactivated delayed fluorescent (TADF) molecules.
 9. The composition ofclaim 5 wherein the blue photoluminescent material is phosphorescent.10. The composition of claim 1, wherein the one or more monomerscomprise (meth)acrylate monomers.
 11. A photocured compositioncomprising: a polymer matrix; a blue photoluminescent material mixed inthe polymer matrix, the blue photoluminescent material selected toabsorb ultraviolet light with a maximum wavelength in a range of about300 nm to about 430 nm and to emit blue light with an emission peak in arange of about 420 nm to about 480 nm, wherein the full width at halfmaximum of the emission peak is less than 100 nm, and thephotoluminescence quantum yield is in a range of 5% to 100%; one or moremonomers; and components of a photoinitiator that initiatedpolymerization to form the polymer matrix.
 12. The composition of claim11, wherein the polymer matrix comprises a polyacrylate.
 13. Thecomposition of claim 11, wherein the blue photoluminescent material isan organic material, an organometallic material, or a polymericmaterial.
 14. The composition of claim 11, wherein the bluephotoluminescent material is fluorescent.
 15. The composition of claim11 wherein the blue photoluminescent material is phosphorescent.
 16. Amethod of fabricating a light emitting device, comprising, comprising:dispensing a first photo-curable fluid over a display having a backplaneand an array of ultraviolet light emitting diodes electricallyintegrated with backplane circuitry of the backplane, the firstphoto-curable fluid including a blue photoluminescent material selectedto absorb ultraviolet light with a maximum wavelength in a range ofabout 300 nm to about 430 nm and to emit blue light with an emissionpeak in a range of about 420 nm to about 480 nm, wherein the full widthat half maximum of the emission peak is less than 100 nm, and thephotoluminescence quantum yield is in a range of 5% to 100%, one or moremonomers, and a photoinitiator that initiates polymerization of the oneor more monomers in response to absorption of the ultraviolet light;activating a first plurality of light emitting diodes in the array oflight emitting diodes to illuminate and polymerize the one or moremonomers to form a first color conversion layer over each of the firstplurality of light emitting diodes to convert light from the firstplurality of light emitting diodes to blue light, each color conversionlayer including the blue photoluminescent material mixed in a polymermatrix; and removing an uncured remainder of the first photo-curablefluid.
 17. The method of claim 16, wherein the one or more monomerscomprise (meth)acrylate monomers.
 18. The method of claim 16, whereinthe composition comprises: about 0.1 wt % to about 10 wt % of the bluephotoluminescent material; about 0.5 wt % to about 5 wt % of thephotoinitiator; and about 1 wt % to about 90 wt % of the one or moremonomers.
 19. The method of claim 16, wherein the composition furthercomprises a solvent, and the method includes evaporating the solvent.20. The method of claim 16, wherein the blue photoluminescent materialis an organic material, an organometallic material, or a polymericmaterial.